The turbojets conventionally found nowadays in the field of civil aviation are two-spool bypass turbojets. Nevertheless, because of the ever-increasing constraints on operating costs, closely tied to the cost of fuel, which is nowadays very high, new projects have been proposed for turbojets that benefit from smaller specific consumption.
One promising option consists in fitting the turbojets with speed reduction gearing interposed between the low pressure compressor and the fan: in this way, it is possible to increase the speed of rotation of the low pressure spool, thereby increasing the overall efficiency of the turbojet, while reducing the speed of the fan, thereby enabling the diameter of the fan to be increased and thus enabling the bypass ratio of the engine to be increased, while conserving a peripheral speed at the tips of the blades that is acceptable for limiting the occurrence of aerodynamic disturbances that, in particular, generate noise.
Such a bypass turbojet with reduction gearing is shown in FIG. 1, in section on a vertical plane containing its main axis A. From upstream to downstream it comprises a fan 2, reduction gearing 3, a low pressure compressor 4, a high pressure compressor 5, a combustion chamber 6, a high pressure turbine 7, and a low pressure turbine 8.
In such a turbojet 1 with reduction gearing, the high pressure turbine 7 drives the high pressure compressor 5 via a high pressure shaft 9. The low pressure turbine 8, also referred to as a fast turbine, drives the low pressure compressor 4 via a low pressure shaft 10. The fast turbine 8 also drives the fan 2 via the speed reduction gearing 3. In this way, the fan 2 is driven at reduced speed, which is favorable from an aerodynamic point of view, while the low pressure turbine 7 can operate at higher speed, which is favorable from a thermodynamic point of view.
As shown in FIG. 2, the reduction gearing 3 may be an epicyclic gear train having a ring 31, a sun gear 32, and planet gears 33. The planet gears 33 are mounted to rotate on spindles 34 of a planet carrier 35: each planet gear 33 thus rotates about the axis F of the corresponding spindle 34. The bearings 36 between the planet gears 33 and their respective spindles 34 may be smooth, i.e. without any rolling mechanism, in which case they have a film of oil under pressure serving to lubricate and cool the bearings 36. An example of such an oil distribution system is given in the French patent application filed under the No. 13/58581.
In a conventional configuration, the ring 31 is fastened to the casing 60, the planet carrier 35 is coupled to the fan shaft 2a, thereby driving the fan 2, and the sun gear 32 is coupled to one end 10a of the low pressure shaft 10.
While the turbine engine is in operation, because the sun gear 32 rotates and because the ring 31 is stationary, the planet gears 33 are driven over a path combining rotation about the axis of rotation A of the epicyclic gear train and rotation about the axes F of their respective spindles 34: under such circumstances, the spindles 34 and the planet carrier 35 as a whole are driven in rotation about the axis of rotation A of the epicyclic gear train.
It can thus be understood that the forces driving the planet gears 33, together with centrifugal force, and to a lesser extent the force of gravity, lead to the planet gears 33 being moved off-center relative to the spindles 34. In particular, as a result of the driving force, the oil film is observed to be pinched behind each spindle 34.
This off-centering then has the consequence of increasing the risk of a planet gear 33 coming into contact with its spindle 34, thereby damaging the bearing.
To remedy that phenomenon, reduction gear units have been proposed in which non-uniform oil distribution is provided within the bearing in order to provide a greater quantity of oil in the zones that are the most exposed to the risk of friction.
Nevertheless, in general, such non-uniform distribution of oil is either a static correction, which therefore does not take account of the genuine state of the system, or else a dynamic correction, but requiring the provision of sensors, actuators, and electronic controllers, thereby greatly increasing the complexity and the cost of the system.
There therefore exists a real need for a mechanical assembly that is free, at least in part, from the drawbacks inherent to the above-mentioned known configurations.